As shown in FIG. 6, prior art failure diagnostic apparatus comprises a plurality of failure detecting devices 1a-1z, each of which failure detecting devices generates various voltages according to the state of failure detected. Diverse voltage levels generated by the respective failure detecting devices 1a-1z are delivered to input port of a microcomputer 4, each of the voltages at a different time. From the level of voltage and the time of input a signal is discernible as to which failure detecting device it is from, and from the voltage content of the failure is judged; the results of judgment is displayed on a Light Emitting Diode 5, or a sole display device, indicating the content of failure with the number of blinking.
In prior art failure diagnostic apparatus, however, the accurate designation of the place of failure was difficult because the output of failure detecting device is inputted direct to microcomputer 4 and the voltage level and the time fluctuate depending on the state of failure.
In the prior art apparatus shown in FIG. 7, disclosed in the Japanese Patent Laid-Open Application No.6-290378, at first a control unit 11 issues series signals ordering slave devices 12, 13, 14 and 15 to detect failure; responding to the series signals the slave devices 12, 13, 14 and 15 output their respective failure data. A failure place specifying unit 19 receives the failure data and conducts diagnosis; judging the place of failure from the timing at which one of the series signals the failure data was outputted, and the content of failure from the level of voltage.
Among the problems prior art procedures have is the difficulty in accurately locating a place of failure; because in the prior art procedures the voltages from failure detecting devices are delivered direct to the input port of a microcomputer, furthermore, the failure diagnosis is dependent on dual factors, viz. the voltage level and the time, and the voltage level and the time fluctuate depending on the state of failure.
According to what is disclosed in the Japanese Patent Publication No. 6-290378 the content of failure and the place of failure are diagnosed based on failure data generated in response to an order in the form of series signals issued from outside; which means that the failure diagnosis is not available without receiving an order in the form of series signals from the outside.